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fix assumed termination measure #329

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merged 1 commit into from
Apr 19, 2024
Merged

fix assumed termination measure #329

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4 changes: 3 additions & 1 deletion pkg/slayers/scion.go
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Original file line number Diff line number Diff line change
Expand Up @@ -1078,9 +1078,11 @@ func (s *SCION) upperLayerChecksum(upperLayer []byte, csum uint32) uint32 {

// (VerifiedSCION) The following function terminates but Gobra can't
// deduce that because of limited support of bitwise operations.
// @ decreases _
// @ decreases
func (s *SCION) foldChecksum(csum uint32) (res uint16) {
// @ decreases csum
for csum > 0xffff {
// @ b.FoldChecksumLemma(csum)
csum = (csum >> 16) + (csum & 0xffff)
}
return ^uint16(csum)
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6 changes: 6 additions & 0 deletions verification/utils/bitwise/bitwise-eqs.gobra
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Expand Up @@ -68,3 +68,9 @@ ensures 3 & 1 == 1
ensures 3 & 2 == 2
decreases
pure func InfoFieldFirstByteSerializationLemmas() bool

ensures csum > 0xffff ==>
let newCsum := (csum >> 16) + (csum & 0xffff) in
newCsum < csum
decreases
pure func FoldChecksumLemma(csum uint32) struct{}
6 changes: 6 additions & 0 deletions verification/utils/bitwise/proofs.dfy
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Expand Up @@ -162,4 +162,10 @@ lemma FUint32AfterFPutUint32(v: bv32)
lemma FPutUint32AfterFUint32(b0: bv8, b1: bv8, b2: bv8, b3: bv8)
ensures var v := FUint32Spec(b0, b1, b2, b3);
FPutUint32Spec(v) == (b0, b1, b2, b3)
{}

lemma FoldChecksumLemma(csum: bv32)
ensures csum > 0xffff ==>
var newCsum := (csum >> 16) + (csum & 0xffff);
newCsum < csum
{}
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